Lithium secondary batteries, e.g., lithium-ion secondary batteries have a high energy density per unit volume, and thus are used as power sources in various portable electronic devices such as notebook computers, camcorders, cameras, and personal digital assistants (PDAs), as well as cellular phones. However, if temperature of a battery in an overcharged or fully charged state rises, the battery can become swollen, there by resulting in explosion at about 150° C. Battery explosions can be accompanied by fire in some cases. Furthermore, when the skin of a consumer is exposed to a poisonous gas generated from batteries, skin irritation may be caused, and in serious cases, inflammation or breathing trouble may occur. Battery explosions are well known in the art. However, this phenomenon is caused by very abrupt temperature elevation, and thus, there is no reliable solution to prevent the risk of explosion.
FIG. 1 is a photographic image of a conventional lithium-ion secondary battery which exploded at a temperature of about 150° C. A battery explosion takes place when gas within the battery is released from a safety vent. In FIG. 1, a deep hole 10 was formed around the safety vent of the battery. Just before an explosion, a battery is extremely swollen compared to the original size of the battery. That is, a battery explosion causes gas release from a safety vent when the internal pressure of the battery reaches a predetermined level at which the outer shell of the battery, extremely swollen by an increase of the internal pressure of the battery, cannot withstand the internal pressure.
FIG. 2A illustrates voltage characteristics with respect to time and temperature in a lithium-ion secondary battery used in cellular phones. The lithium-ion secondary battery has a standard charge voltage of 3.7 V, a full charge voltage of 4.2 V, and a charge current of 830 mAh (standard) and 1,900 mAh (large capacity).
Referring to FIG. 2A, as the temperature exceeded about 100° C., voltage was gradually lowered due to the internal breakdown of the battery. At about 123° C., a voltage for a power-down mode dropped abruptly, and a cellular phone did not work due to low current less than 100 mA, and thus a slight voltage increase was observed. At about 164° C., voltage was abruptly lowered. At about 173° C., a battery explosion occurred.
Due to the battery explosion, voltage was hardly observed at about 173° C.
As the internal temperature of the battery exceeded about 100° C., voltage was lowered by the internal change of the battery. At about 123° C., the cellular phone was disconnected from a wireless network and even turned off. At about 164° C., the voltage reached about 0 V. At about 173° C., an explosion occurred. That is, at about 90° C., a fully charged battery undergoes a large internal change, and thus, becomes so unstable that a stable power supply cannot be guaranteed. At a temperature greater than 90° C., the battery cannot be used any more because it has already run dead.
FIG. 2B is a graph illustrating current characteristics with respect to time and temperature in the same battery as used in FIG. 2A. Referring to FIG. 2B, until a temperature of about 123° C. was reached, the battery was normally operated during a power-down mode (5 mA) and a normal mode (100 mA). Above 123° C., battery characteristics became lowered. That is, at about 123° C., the power supply to the battery was stopped, and thus, only a small current of 13 μA flowed through the battery. At about 164° C., the current dropped to about zero. Until a temperature of 173° C. was reached, zero current was detected. At 173° C., a battery explosion occurred. At this time, a discharge amount was about 0.03%. That is, at about 123° C., the current dropped abruptly and the power supply to the battery was stopped. At about 173° C., a battery explosion occurred.
In view of the above-described battery explosion phenomenon, lithium-ion secondary batteries are provided with safety systems such as positive temperature coefficients (PTCs) on their insides and protection circuits on their outsides, to protect them from the risk of explosion or fire due to overcurrent, overdischarge, or overheating. The PTCs interrupt a current flow toward batteries when the temperature of the batteries rises to a predetermined level. At this time, the interrupting current is about 3 A or more. That is, overcurrent flows through the batteries until a current of about 3 A is reached. Also, as the temperature of the batteries increases, the batteries become unstable. The above-described safety systems work only in a temperature range of −20° C. to 60° C. in which cellular phones are normally used and operated. Thus, there is no safety apparatus capable of delaying or preventing the explosion of lithium secondary batteries exposed to high temperatures except a safety vent for releasing gas fully filled in the batteries.